A steam turbine is made up of a large number of turbine rotor blades implanted in a circumferential direction of a rotor and stator blades (nozzles) fixed to a turbine casing. A combination of a turbine rotor blade and a stator blade forms a turbine stage. By arranging a plurality of turbine stages on disks, a steam turbine is formed. The turbine rotor blades and the disks are formed of, for example, a material (for example, high-strength copper) with high tensile strength of 500 kg/mm2 or higher in order to endure centrifugal force during operation, and are coupled by a fork structure for example. This material with high tensile strength is sensitive to stress-corrosion cracking (hereinafter referred to as “SCC”).
Large centrifugal force acts on a coupling portion of a turbine rotor blade and a disk accompanying rotation of the steam turbine, and also steam enters this coupling portion via gaps. As a result, SCC occurs in this coupling portion due to aging caused by accumulation of corrosive products such as Na and Cl contained in the steam.
In recent years, to improve efficiency of steam turbines, there is a tendency for employing a turbine blade with a long blade length for the purpose of decreasing exhaust loss in a low-pressure final stage. However, accompanying increase in blade length, the centrifugal force acting on this blade increases. Accordingly, the coupling portion of a blade and a disk needs to have sufficient strength against low-cycle fatigue accompanying start and stop and high-cycle fatigue under high mean stress and in a corrosive environment.
Accordingly, there has been proposed a method to perform shot peening on the surface of a pin or the like for coupling a turbine rotor blade and a disk to add compressive residual stress, in order to suppress occurrence of SCC.
However, the above-described prior art shoots a large amount of projectiles to an operating range by shot peening, and thus how it is performed uniformly within this operating range is a matter of probability. Therefore, when the shot peening is applied to both the pin and a pin hole, if the shot peening is performed unevenly, a tolerance between the diameter of the pin and the diameter of the pin hole surpasses an allowable range, and there is a possibility that precise coupling is impaired.